Control circuits of this kind, although having many other applications, are commonly employed in motion picture theatres and other auditoriums equipped with public address systems or other forms of amplified sound equipment. By their use a person, or persons, at different parts of the theatre may determine the optimum volume of sound for the comfort of patrons. This has been achieved by locating a multiplicity of controlling devices at individual positions throughout the theatre and providing for control of the sound amplitude at any instant only at a single device which has been selected by the operator and identified. Hitherto, equipment of this nature has employed electro-mechanical components and has been expensive, cumbersome and not sufficiently flexible in use.